This invention relates to water treatment systems and, in particular, to treatment of waterways and aquatic systems such as wetlands which may become subject to contamination with pollutant rich, for example nutrient rich, run off and other effluent waters. The invention further relates to an improved water treatment method.
Waterways, such as wetlands, rivers, creeks, flood prone areas and so on are subject to various sources of pollution. Significant among these are rain or stormwater flows which may become contaminated with various contaminants and pollutants which exist in the wider environment. Key among these pollutants are the nutrients phosphorus and nitrogen present in agricultural fertilisers. Other sources of pollution include particulate matter, other organochemicals and metals.
It has been recognised that stormwater is especially polluting in the early time phase following a storm. International Patent Application No. WO 97/41068, assigned to CSF Treatment Systems Inc, identifies a xe2x80x9cfirst flushxe2x80x9d phenomenon in which high levels of contaminants are present in effluent water which sooner or later will enter waterways within the environment.
These problems have been addressed in diverse and not totally effective ways in the past. Firstly, processes and apparatus have been developed to remove contaminants where they enter the drainage system. Thus special drainage ways may be designed containing suitable reactors such as baskets containing adsorbing and adsorbent materials, especially carbonaceous materials. The cost of equipping all entry points to the drainage system in this way would be extreme.
Further downstream, processing facilities may be located to treat the bulk of water collected within the drainage system. Many different types of facility have been designed. These facilities may operate using combinations of sedimentation, flocculation, filtration and chemical reaction to remove various pollutants from the pollutant water.
There are limitations to such processing. Apart from expense, the methodology suffers the critical constraint that only water collected within a municipal drainage system will be treated. Run off, however, is a much more diffuse source of waterway contamination than this and it can be estimated that much more run off escapes xe2x80x9cformalxe2x80x9d treatment than is actually treated. Furthermore, treatment facilities are likely to become less efficient when surge waterflows are generated following heavy rainfall and storms. Therefore, treatment still further downstream is required.
Downstream processing is driven by the need to prevent phenomena such as algal blooms. Such blooms are highly deleterious to aquatic environments by destruction of ecosystems and reduction of the social amenity of aquatic resources. Algal blooms may be correlated with nutrient levels contained in run off entering the aquatic environment.
The most commonly encountered methodology of downstream treatment especially in wetlands is nutrient removal by plant growth. Reed beds may be planted to remove nutrients. Such processes are made difficult by the requirements of efficient plant growth, namely the requirement to provide a good soil medium in which the plants can grow. Further, once the system has reached a steady state, nutrient take-up plateaus and nutrient bypassing of the reed beds becomes an issue. The only way to address this problem is to crop the reeds or expand the treatment area. Again cost, available land and labour constraints will hinder this process.
It is a first object of the present invention to provide a water treatment system and method that will enable waterways to be treated for removal of pollutants and contaminants such as nutrients present in run-off and other sources of water discharged into the environment.
It is a second object of the present invention to allow treatment of intermittent and/or variable flow waterways.
With these objects in view, the present invention provides, in a first aspect, a water treatment system including:
(a) a water permeable wall portion defining a water treatment portion of a waterway to be treated; and
(b) a treating portion containing a treating material and retained in a desired position relative to the permeable wall portion by retaining means wherein water to be treated permeates through said wall portion, said retaining means and said treating portion for removing pollutants contained therein prior to discharge of water from the system.
The water treatment portion defines at least part of a catchment of water to be treated. The catchment may be further defined by a liner for further confinement of the water to be treated. Such a liner may be impermeable to prevent polluted water from entering the environment and may be located at the base of the waterway to be treated. A bed of treating material such as an adsorbent material may be laid over the liner to provide additional treatment to that possible in the treating portion.
The catchment need not be full or even partially full of water at any given time. The catchment may only contain water following rainfall, including storm, events. Flow through the catchment may be intermittent and/or variable and inlet flow may be controlled by man-made devices such as sluices, dams and like means. The water treatment system may be installed when conditions are dry without any water flow through the catchment at all. Accordingly, the term xe2x80x9cwaterwayxe2x80x9d where used in this specification includes rivers, creeks, drains, wetlands, floodways, flood-prone areas and the water treatment portion may be a diverted flow of water from any such waterway.
The wall portion may be an embankment, bund or dike made with a portion of a suitably permeable material. Water to be treated may, in one embodiment, permeate or flow through this portion of the wall portion to reach the treating portion and complex piping and regeneration systems are most advantageously and preferably avoided. Alternatively, water to be treated may flow through the wall following passage through the treating portion. The permeable wall portion could include permeable portions in a substantially impervious structure.
The treating portion comprises a treating material or substance which may adsorb and/or adsorb pollutants, an adsorbent material. Alternatively, or additionally, the material may react with pollutants in the water to convert them into a safer form for later safe disposal. The treating material may be the same as that used in the treating portion. Mixtures of suitable treating materials may be employed.
Any suitable adsorbent or adsorbent may be used for the application. In particular, the system is suited to treatment of wetland systems where the waterway may be contaminated by nutrient rich run-off. Such run-off may contain high phosphorus levels and high phosphorus capacity is desirable in such treating materials. The Applicant has found that granulated slag, most especially that from iron blast furnaces, is especially suitable having high capacity for adsorption of phosphorus.
Whatever the treating material used, the permeability and hydraulic properties must be such as to allow efficacious removal of pollutants without the need for involvement of expensive pumping systems. As permeability is dictated, in part, by the particle size distribution of the treating material, the particle size distribution is selected to allow the desired flowrates to be achieved without overflow from the system or requirement for expensive pumping systems to develop a necessary head to cause flow through the treating material. Adsorbent material particle sizing may be necessary to achieve this objective.
The body of the wall portion may be substantially comprised of a porous or permeable material. For example, the wall portion may be made up of rubble, spall, pebbles, rocks, bricks or like materials which may have a porosity higher than concrete. The wall material should be inert and should not be such as to itself pollute water passing through the wall portion. The wall material may itself have treatment properties, such as adsorbency, analogous to those of the treating material.
The treating portion may be retained in a desired position relative to the body of the wall portion by a retaining means or structure which itself is permeable to polluted water flow while retaining the treating material within the wall portion. Suitable means or structures may be constructed using plastic, polymeric, fabric or textile membranes which allow water through flow but which are fabricated with pore size small enough to retain the treating material in the wall portion. In a simple form, the body of the wall portion may serve a retaining function by overlaying and weighing down a retaining structure or means into position for treatment of a water flow. The retaining structure may be a water permeable envelope or xe2x80x9cbagxe2x80x9d which is located by the weight of the body of the wall portion. The envelope may advantageously be fabricated from a geotextile material.
The retaining structure or means may be such as to allow development of a biologically active microflora which may also be involved in the pollutant removal process. A slime or film containing the microflora may, for example, develop along a surface or surfaces of the retaining structure such as an envelope or bag which may be formed from a geotextile.
The system advantageously caters for treatment of intermittent and variable flows within the waterway to be treated. The wall portion should be capable of containing water for treatment under expected conditions within the waterway. Design parameters may be set according to worst case scenarios over a preset time period, for example ten year flood peak conditions and so on.
The containing wall portion may be provided with a spillway to allow excess water flows to be discharged from the system in such a way as to cause minimal damage to the structure. Where the bulk of the wall portion is comprised of a rock or rubble material this may be sealed, in the region of the spillway, with concrete or other suitable structural material which defines a spillway through which floodwaters may pass without severely damaging the impounding or containing wall portion.
The wall portion may be designed to create any desired form of treating portion. In a basic form, the wall portion may simply comprise a dam located to intercept a waterway to be treated. In the case of wetland treatment, more complex arrangements may be necessary as contaminated water may diffuse from the wetland around its periphery subject to local topography. In that case, the wall portion which may take the form of a dike, embankment or bund, may be arranged about a portion or all of the periphery of the wetland where contaminated waterflows are sufficiently substantial to merit treatment. Entry of water to the confined area may be controlled by man-made devices. The containing wall portion need not, of course, be located along the periphery of a wetland, it may be located to define a treating portion along a line or xe2x80x9ccontourxe2x80x9d selected by the environmental engineer or scientist to achieve maximum benefit in terms of pollutant or contaminant removal relative to the cost of construction.
Land area available for the treatment system may be limited. Accordingly, the treating portion may be designed to be suitably confined within the area. Thus a tortuous or serpentine alignment or contour may be chosen for the wall portion which allows maximum treatment interface per unit of available land area.
The water treatment system may be located in a natural or excavated basin which may allow collection of treated water. This may be discharged at designated exit points, for example including pipes and culverts.
In a further aspect, the present invention provides a method of treating water to remove contaminants or pollutants including:
(a) confining water to be treated in a treatment portion containing a treating material with a water permeable wall portion and;
(b) causing at least a portion of the water to flow through a treating portion retained in a desired position relative to the permeable wall portion by retaining means such that pollutants are removed therefrom.
The method may be practiced in systems as described hereinabove and may adopt each of those aspects previously described.
An advantage of the system and method of the present invention is simplicity of construction and an expected higher efficiency of pollutant, especially nutrient, removal than encountered in prior art treatment methodology.